In high-frequency/high-capacity optical fiber communication systems, optical modulators embedded with waveguide-type optical modulation elements are frequently used. Among these, optical modulation elements in which LiNbO3 (hereinafter, also referred to as LN) having an electro-optic effect is used for substrates cause only a small light loss and are capable of realizing broad optical modulation characteristics and are thus widely used for high-frequency/high-capacity optical fiber communication systems.
In an optical modulation element in which this LN is used, Mach-Zehnder-type optical waveguides, RF electrodes for applying radio frequency signals, which are modulation signals, to the optical waveguides, and bias electrodes for performing a variety of adjustments for favorably maintaining modulation characteristics in the waveguides are provided. In addition, these electrodes provided in the optical modulation element are connected to an external electronic circuit via lead pins or connectors provided in a package case of the optical modulator which accommodates the optical modulation element.
On the other hand, regarding modulation forms in optical fiber communication systems, in response to the recent trend of an increase in transmission capacity, multilevel modulation or transmission formats achieved by incorporating polarization multiplexing into multilevel modulation such as Quadrature Phase Shift Keying (QPSK) or Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) has become mainstream.
Optical modulators performing QPSK modulation (QPSK optical modulators) or optical modulators performing DP-QPSK modulation (DP-QPSK optical modulators) include a plurality of Mach-Zehnder-type optical waveguides having a nested structure and include a plurality of radio frequency signal electrodes and a plurality of bias electrodes (see, for example, Patent Literature No. 1) and thus tend to cause an increase in the sizes of package cases of the optical modulators, which creates a strong demand for, particularly, size reduction.
As a measure for satisfying the above-described demand for size reduction, an optical modulator in which push-on-type coaxial connectors provided in the package case of an optical modulator of the related art as interfaces of the RF electrodes are replaced by the same lead pins as the interfaces of the bias electrodes and a flexible printed circuit (FPC) for connecting these lead pins to external circuit substrates is added is proposed.
For example, in a DP-QPSK optical modulator, an optical modulation element constituted of four Mach-Zehnder-type optical waveguides respectively having RF electrodes is used. In this case, four push-on-type coaxial connectors provided in the package case of the optical modulator inevitably increase the size of the package case, but the use of the lead pins and an FPC instead of the coaxial connectors enables size reduction.
In addition, since the lead pins in the package case of the optical modulator and a circuit substrate on which electronic circuits for causing modulation operations in the optical modulator are mounted are connected to each other through the FPC, it is not necessary to perform the excess length treatment of coaxial cables used in the related art, and it is possible to decrease the installation space of the optical modulator in optical transmission devices.
An FPC which is used in the optical modulator is produced using, for example, a flexible polyimide-based material as a substrate (hereinafter, an FPC substrate), and a plurality of through-holes provided in the vicinity of one end portion are respectively electrically connected to individual pads provided in the other end portion through wire patterns. In addition, a plurality of the lead pins protruding from the bottom surface or side surfaces of the package case of the optical modulator are respectively inserted into the plurality of through-holes and are fixed by means of, for example, soldering and electrically connected to the through-holes, and the plurality of pads are fixed by means of, for example, soldering and connected to the circuit substrate. Therefore, radio frequency signals that are supplied from the pads on the circuit substrate are supplied to the corresponding RF electrodes in the optical modulation element through the respective corresponding through-holes and lead pins, whereby high-frequency optical modulation is performed.
As described above, the optical modulator in which an FPC is used enables the size reduction of the package case and also a decrease in the installation space of the optical modulator on the circuit substrate and is thus capable of significantly contributing to the size reduction of optical transmission devices.
FIGS. 9(a) to 9(c) are views illustrating the constitution of an optical modulator of the related art which includes such an FPC, and FIG. 9(a), FIG. 9(b), and FIG. 9(c) are a top view, a front view, and a bottom view of the optical modulator, respectively. The optical modulator 900 includes an optical modulation element 902, a package case 904 accommodating the optical modulation element 902, a flexible printed circuit (FPC) 906, an optical fiber 908 for making light incident on the optical modulation element 902, and an optical fiber 910 guiding the light output from the optical modulation element 902 to the outside of the package case 904.
In the package case 904, four lead pins 920, 922, 924, and 926 respectively connected to four RF electrodes (not illustrated) of the optical modulation element 902 are provided, and the lead pins 920, 922, 924, and 926 are inserted into through-holes 1020, 1022, 1024, and 1026 described below, which are provided in the FPC 906, and are fixed by means of, for example, soldering and electrically connected to the through-holes.
FIGS. 10(a) and 10(b) are views illustrating the constitution of the FPC 906. FIG. 10(a) is a view illustrating the constitution of one surface of the FPC 906 (for example, a surface illustrated in FIG. 9(c), referred to as a “front surface” here), and FIG. 10(b) is a view illustrating the constitution of the other surface (referred to as a “back surface”) of the FPC 906. On the front surface illustrated in FIG. 10(a), four pads 1010, 1012, 1014, and 1016 are provided in parallel in the vicinity of one side 1000 on the lower side in the drawing, along the direction of the side 1000. In addition, four through-holes 1020, 1022, 1024, and 1026 are provided in parallel on a side of the other side 1002 opposite to the side 1000, for example, along the direction of the side 1002. Further, the four pads 1010, 1012, 1014, and 1016 are electrically connected to the through-holes 1020, 1022, 1024, and 1026 through wire patterns 1030, 1032, 1034, and 1036, respectively.
On the other hand, a ground pattern 1040 (a hatched portion shown in the drawing) is formed on the back surface illustrated in FIG. 10(b). In order to avoid an electrical contact between the ground pattern 1040 and the through-holes 1020, 1022, 1024, and 1026, a conductor constituting the ground pattern 1040 is removed in a circular shape in the peripheral portions of the through-holes 1020, 1022, 1024, and 1026.
Each of the wire patterns 1030, 1032, 1034, and 1036 formed on the front surface illustrated in FIG. 10(a) may be designed such that, for example, characteristic impedance is set to 50 Ω by the ground pattern 1040 formed on the back surface with the substrate of the FPC 906 interposed therebetween.
The four pads 1010, 1012, 1014, and 1016 are respectively fixed by means of, for example, soldering and electrically connected to the pads in the external circuit substrates, whereby the RF electrodes in the optical modulation element 902 included in the optical modulator 900 and electronic circuits constituted on the circuit substrates are electrically connected to each other and the optical modulator 900 is driven. Meanwhile, the shape of the FPC 906 is generally a horizontally long rectangular shape having a short side in a signal transmission direction as illustrated in the drawing in order to extremely shorten wire patterns and suppress microwave loss at a low level, and is a rectangular shape which is approximately 20 mm or less in the long side direction and approximately 10 mm or less in the short side direction in a case where the FPC includes the four pads 1010, 1012, 1014, and 1016 as in the example illustrated in the drawing.
FIGS. 11(a) and 11(b) are views illustrating an example of a state where the optical modulator 900 is connected to a circuit substrate on which an electronic circuit is constituted. FIG. 11(a) is a view of the optical modulator 900 seen from above (a direction in which the surface illustrated in FIG. 9(a) is seen), and FIG. 11(b) is a cross-sectional view in a direction of BB line in FIG. 11(a). Meanwhile, the internal constitution of the optical modulator 900 in FIG. 11(b) is not illustrated.
An electronic circuit including a driving circuit 1104 for driving the optical modulation element 902 of the optical modulator 900 is constituted on the circuit substrate 1100, and the optical modulator 900 and the circuit substrate 1100 are fixed to, for example, a base 1102 inside a package case of an optical transmission device. As illustrated in FIG. 11(a), the FPC 906 of the optical modulator 900 extends from the connection portions with the lead pins 920, 922, 924, and 926 toward the left in the drawing and bends slantwise in the left downward direction in the drawing so as to come into contact with the circuit substrate 1100 at the left end portion as illustrated in FIG. 11(b), whereby the pads 1010, 1012, 1014, and 1016 of the FPC 906 are fixed by means of, for example, soldering and electrically connected to the pads 1110, 1112, 1114, and 1116 on the circuit substrate 1100 (FIG. 11(a)).
Meanwhile, the connection between the ground pattern 1040 and the optical modulator 900 can be performed, for example, by providing a lead pin for grounding (not illustrated) connected to a ground pattern formed on the optical modulation element 902 in the package case 904, providing a hole (not illustrated) engaging with the lead pin for grounding in the FPC 906, and inserting the lead pin for grounding into the hole so as to be soldered to the ground pattern 1040. In addition, the connection between the ground pattern and the circuit substrate 1100 can be performed, for example, by providing a conductor pin (not illustrated) brazed to the ground pattern 1040 in the FPC 906 and connecting the conductor pin and the ground pattern on the circuit substrate 1100 by soldering.
In the optical modulator 900 having the above-described constitution, the driving circuit 1104 provided on the circuit substrate 1100 and the lead pins 920, 922, 924, and 926 of the optical modulator 900 are connected to each other through the wire patterns 1030, 1032, 1034, and 1036 having predetermined characteristic impedance. Thereby, the output impedance of the driving circuit 1104, the input impedance of the optical modulator 900, and characteristic impedance of the wire patterns 1030, 1032, 1034, and 1036 are matched to each other, whereby it is possible to efficiently propagate a radio frequency signal from the driving circuit 1104 to the optical modulator 900 in principle.
However, a mismatching portion of impedance is actually generated in a path of a radio frequency signal from the driving circuit 1104 to the optical modulator 900 due to manufacturing variations of the FPC 906 or the like, and the radio frequency signal may be reflected due to the impedance mismatching. In particular, such impedance mismatching easily occurs in soldering connection portions between the lead pins 920, 922, 924, and 926 provided in the package case 904 and the through-holes 1020, 1022, 1024, and 1026 provided in the FPC 906.
That is, in general, outer diameters of the lead pins 920, 922, 924, and 926 and inner diameters of the through-holes 1020, 1022, 1024, and 1026 are not fixed at all times between products, and the lead pins and the through-holes are manufactured with a certain dimension tolerance. For this reason, variations in distances between the outer surfaces of the lead pins 920, 922, 924, and 926 and the inner surfaces of the through-holes 1020, 1022, 1024, and 1026, variations in the shape of soldering, and the like occur in the soldering connection portions between the lead pins 920, 922, 924, and 926 and the through-holes 1020, 1022, 1024, and 1026, and impedance mismatching easily occurs due to the variations.
When such impedance mismatching occurs, a portion of power of a radio frequency signal passing through a portion in which the mismatching occurs is reflected from the portion toward a direction opposite to a propagation direction, and is incident on, for example, the output of the driving circuit 1104. As a result, operation in the driving circuit 1104 becomes unstable, and an unstable phenomenon such as noise occurs in the radio frequency signal output from the driving circuit 1104 in some cases, which may result in a problem in transmission quality of an optical system using the optical modulator 900.